1. Field of the Invention:
The present invention relates to a gas barrier laminated film useful as a packaging material, which possesses transparency and is excellent in gas barrier property against oxygen, water vapor, etc., especially in gas barrier property at high humidity as well as a process for producing same. More particularly, the present invention relates to a gas barrier laminated film wherein a composition comprised of an ethylene-vinyl alcohol copolymer and a (meth)acrylic polymer is formed as a gas barrier layer on at least one surface of a film substrate as well as a process for producing same.
2. Description of the Prior Art:
In recent years, a transparent gas barrier film having a film base coated thereon an inorganic oxide such as silicon dioxide, alumina or the like oxide by way of vacuum vapor deposition, spattering, ion-plating, chemical vapor deposition, and the like methods has attracted public attention as barrier material against oxygen, water vapor, etc. In general, such transparent gas barrier film is a biaxially stretched polyester film excellent in transparency and rigidity as a substrate coated thereon an inorganic oxide merely by way of vapor deposition, and so its vapor deposition layer is weak as such against abrasion on use. In case of using it as a packaging film, therefore, there may be a fear that cracks are formed in the inorganic oxide by friction or elongation in the course of printing or lamination as a post-processing or during packaging of contents, thus causing deterioration of gas barrier property.
Proposed heretofore are a method of laminating gas barrier polyvinyl alcohol, ethylene-vinyl alcohol copolymer onto the biaxially stretched film substrate (for example, Japanese Laid-open Patent Appln. No. Sho. 60-157830, especially claims thereof), a method of coating a biaxially stretched film with a composition comprising polyvinyl alcohol and poly(meth)acrylic acid (for example, Japanese Patent 3,203,287, claim 1), a method of laminating gas barrier polyvinyl alcohol on a thin film of a metal oxide (Japanese Laid-open Patent Appln. No. Hei. 6-316025, claim 1), a laminated film prepared by applying onto a vapor deposition layer comprising an inorganic compound a coating agent comprising predominantly of (a) at least one alkoxide and/or a hydrolyzate thereof and (b) an aqueous solution or a water/alcohol mixed solution of at least one tin chloride (Japanese Patent 2,790,054, claim 1), a laminated film manufactured by applying a coating composition comprising a specific organosilane, a fluorine-containing polymer having silyl groups and an organopolysiloxane onto a substrate (Japanese P2000-63752A, claims 7 and 11), and a gas barrier film manufactured by applying a coating agent comprising a polyvinyl alcohol resin and a metal alcoholate onto a substrate (Japanese P2002-173631A, claims 1 and 11).
Known as a technology paying attention to a substrate layer is a gas barrier laminate wherein a film formed from a mixture of polyvinyl alcohol and poly(meth)acrylic acid or a partially neutralized product thereof is placed adjacent to a layer of a thermoplastic resin (Japanese Laid-open Patent Appln. No. Hei. 7-205379). No special limitation is set for the thermoplastic resin but there are mentioned, for example, polyethylene terephthalate (PET), a polyamide such as nylon 6, nylon 66, nylon 12, nylon 6-nylon 66 copolymer, and nylon 6-nylon 12 polymer; a polyolefin such as a low density polyethylene, a high density polyethylene, a linear low density polyethylene, a vinyl acetate copolymer, polypropylene, an ethylene-acrylic acid copolymer, an ethylene-a metal acrylate copolymer, and ethylene-ethyl acrylate copolymer; halogen-containing resin such as polyvinyl chloride and polyvinylidene chloride; and polyphenylene sulfide.
Further proposed is a laminate wherein a polyvinyl alcohol polymer layer having an olefin content of 1˜25 mol % and a decreasing rate of 30% or less in the case being treated with water at 60° C. for a period of 15 minutes is existent on at least one surface of a substrate layer which is a thermoplastic resin having been stretched at least one direction (Japanese P2001-191461A).
It is noteworthy, however, that polyvinyl alcohol-laminated gas barrier films tend to deteriorate in barrier property against oxygen at a high humidity, while those using a composition comprising polyvinyl alcohol and poly(meth)acrylic acid tend to lose transparency at the time of crosslinking due to yellowing or the like phenomenon.
As a biaxially stretched polypropylene film excels in transparency, mechanical strength and rigidity, the film is employed in a wide variety of industrial fields including packaging materials. However, the biaxially stretched polypropylene film has a specific property that it permits transmission of oxygen and water vapor to a minor degree so that the film is used in the form of a composite film wherein one side of the film is usually laminated with a gas barrier layer capable of reducing transmission rate of oxygen or water vapor especially for use in packaging material for food, cosmetics, medicines, etc. with a view to preventing deterioration of materials to be packaged.
A composition comprising polyvinyl alcohol and a (meth)acrylic polymer is excellent in gas barrier property. According to the inventors' investigation, however, it has been found that if polypropylene is used as a thermoplastic resin to be used for substrate, there may be a case that the substrate cannot be laminated as such with the composition by coating.
The laminate disclosed in the above-mentioned Japanese P-2001-191461A is excellent in improvement in water-resistance of the polyvinyl alcohol polymer, gas barrier property and transparency, but involves a problem such that a trouble occurs in the course of coating when the viscosity of the coating liquid is excessively high. If the viscosity of the coating liquid is reduced as a countermeasure of the above defect, the resultant laminate would not comply with stretching at a ratio of around 10 times when an ethylene-vinyl alcohol copolymer having a polymerization degree of 500 or less is used, thus resulting in breakage of the film.